In 1964 Macfarlane and Davie & Ratnoff [Macfarlane R G. An enzyme cascade in the blood clotting mechanism, and its function as a biochemical amplifier. Nature 1964; 202: 498-9; Davie E W, Ratnoff O D. Waterfall sequence for intrinsic blood clotting. Science 1964; 145: 1310-2.] introduced their cascade hypotheses for the process of blood coagulation. Since then, our knowledge of the function of coagulation in vivo has grown. In the last years, the theory of two distinct routes, the so called the extrinsic and intrinsic pathway, that initiate coagulation and converge in a common pathway, ultimately leading to thrombin generation and fibrin deposition, has been revised. In the current model initiation of coagulation occurs when the plasma protease activated factor VII comes into contact and by this forms a complex, with Tissue Factor (TF). This Tissue Factor-FVIIa complex can activate the zymogen FX into its active form FXa, which on his part can convert prothrombin (coagulation factor II) into thrombin (IIa). Thrombin, a key player in coagulation, in turn can catalyze the conversion of fibrinogen into fibrin. Additionally, thrombin activates specific receptors expressed by platelets, which leads to the activation of the latter. Activated platelets in combination with fibrin are essential for clot formation and therefore are fundamental players of normal hemostasis.
The second amplification route is formed by the coagulation factor XI (FXI). It is well confirmed that FXI is, like the other members of the coagulation cascade, a plasma serine protease zymogen with a key role in bridging the initiation phase and the amplification phase of blood coagulation in vivo [Davie E W, Fujikawa K, Kisiel W. The coagulation cascade: initiation, maintenance, and regulation. Biochemistry 1991; 30:10363-70; Gailani D, Broze Jr G J. Factor XI activation in a revised model of blood coagulation. Science 1991; 253:909-12; Kravtsov D V, Matafonov A, Tucker E I, Sun M F, Walsh P N, Gruber A, et al. Factor XI contributes to thrombin generation in the absence of factor XII. Blood 2009; 114: 452-8.3-5]. FXI deficiency usually does not lead to spontaneous bleeding, but is associated with increased risk of bleeding with hemostatic challenges, while the severity of bleeding correlates poorly with the plasma level of FXI. Severe FXI deficiency in humans has certain protective effects from thrombotic diseases [Salomon O, Steinberg D M, Zucker M, Varon D, Zivelin A, Seligshon U. Patients with severe factor XI deficiency have a reduced incidence of deep-vein thrombosis. Thromb Haemost 2011; 105:269-73; Salomon O, Steinberg D M, Koren-Morag N, Tanne D, Seligsohn U. Reduced incidence of ischemic stroke in patients with severe factor XI deficiency. Blood 2008; 111:4113-7]. Yet, a high level of FXI has been associated with thrombotic events [Meijers J C, Tekelenburg W L, Bouma B N, Bertina R M, Rosendaal F R. High levels of coagulation factor XI as a risk factor for venous thrombosis. N Engl J Med 2000; 342:696-701]. Inhibition of FXI has therefore been proposed as a novel approach in the development of new antithrombotics to achieve an improved benefit-risk ratio. Thus, there is still a high medical need for anti-thrombotic, anti-platelet drugs that blocks intravascular thrombosis efficaciously without debilitating hemostasis.